An Efficient Description of the Dynamics of Barotropic Flow

FM Selten

A potentially efficient description of the atmospheric circulation is investigated in the context of a barotropic spectral model, truncated to T21. The model circulation evolves around a realistic winter climate and has a reasonable low-frequency variability. This study is motivated by the observation that the atmosphere continuously generates coherent structures, which perhaps are better represented by Empirical Orthogonal Functions (EOF\'s) than by spherical harmonics. Therefore, we propose to project the dynamical equations onto the dominant EOF\'s. Ambiguities in the formulation of an EOF-model are clarified. Careful attention is paid to the integral constraints of an EOF-model. As a reference for the performance of the EOF-models, a T20-version of the T21-model is used. The T21-model has 231 variables, the T20-version 210. Short-range predictions of the flow of the T21-model by the EOF-model truncated to only 20 EOF\'s turn out to be substantially better than the short-range predictions by the T20-model. The predictions of the EOF-model monotonically improve as more EOF\'s are included. The application of an EOF filter improves the predictions by the T20-model. The systematic effect of the neglected interactions in the truncated EOF-model can be parameterized by a linear damping. The objectively determined damping time-scale turns out to be scale-selective. It is stronger for EOF\'s containing smaller scale structures. With this closure assumption, the T21-climatology and variability are well reproduced by the EOF-model with 20 EOF\'s. The same closure is shown to be inadequate in case of the T20-model.